Apparent Barrier Height Research Articles

Modified approach to modeling barrier inhomogeneity in Schottky diodes

The conventional model describing barrier inhomogeneities in Schottky diodes with a single series resistance leads to many anomalies including crossing over effect in current–voltage–temperature (I–V–T) plots. A new model is therefore proposed here which entails a parallel combination of several diodes, each with its own series resistance. Further, the barrier heights follow a Gaussian distribution function ρ(φ) with a mean barrier height and standard deviation σ. The series resistance is believed to vary inversely with the ρ(φ) value of the concerned barrier height and hence the area occupied. The occurrence of anomalies with the conventional model is examined in depth, and their effective elimination with the proposed model is discussed with the undertaken current simulations. In addition to predicting the increase in apparent barrier height with increase in temperature, decrease in ideality factor with increase in temperature, and elimination of modeling anomalies observed in the conventional model, the model is successful in predicting that apparent barrier height as a function of inverse thermal energy is a quadratic behavior even with a single distribution. This previously required unlikely scenarios involving multiple mean barrier heights and standard deviations with the conventional model. Finally, the description also explains the observed increase in the apparent barrier height with decrease in temperature of a Schottky diode when the values are deduced from the C–V–T data under reverse bias.

Gamma Irradiation Effect on Performance of β-Ga2O3 Metal-Semiconductor-Metal Solar-Blind Photodetectors for Space Applications

The radiation hardness of Ni/Ga2O3/Ni metal-semiconductor-metal (MSM) solar blind photodetectors has been investigated under the exposure of 60Co γ-source. It was observed that the metal contacts were not degraded and the dark current of photodetector was slightly improved from 3.27 × 10−7 A to 1.88 × 10−7 A. The photo to dark current ratio (PDCR) was observed to increase from 5.1 to 14.1 with increasing γ-radiation exposure. The apparent Schottky barrier height (SBH) evaluated from current-voltage characteristics were found to increase with irradiation. The increased SBH was explained using image force induced barrier lowering. The obtained results reveal that the Ga2O3 solar blind photodetectors are relatively less susceptible to radiation environment.

Apparent tunneling barrier height and local work function of atomic arrays.

Spatially resolved measurements of the apparent tunneling barrier height Φapp in scanning tunneling microscopy have been used to estimate variations of the local work function Φ of surface structures. We experimentally show that Φapp can fail as a measure of Φ. The discrepancies are attributed to a kinetic-energy contribution to Φapp. This contribution depends on the lateral extent of the tunneling current filament and, consequently, on the local surface structure.

β-Ga2O3-based metal–oxide–semiconductor photodiodes with HfO2 as oxide

We proposed β-Ga2O3-based metal–oxide–semiconductor (MOS) photodiodes using HfO2 as the oxide. Because of the larger band gap of HfO2 than that of β-Ga2O3 and the type-II band alignment at the HfO2/β-Ga2O3 interface, the MOS photodiodes exhibited almost no reduction in photoresponsivity against 254 nm light near zero external bias compared with Schottky photodiodes. In contrast, reverse leakage currents of the MOS photodiodes were substantially decreased compared with Schottky photodiodes owing to the increase in the apparent barrier height between the metal and the semiconductor. These features of the MOS photodiodes are beneficial for the future development of β-Ga2O3-based avalanche photodiodes without p–n junctions.

Impact of carbon impurities on the initial leakage current of AlGaN/GaN high electron mobility transistors

We have systematically studied the origin of high gate-leakage currents in AlGaN/GaN high electron mobility transistors (HEMTs). Devices that initially had a low gate-leakage current (good devices) are compared with ones that had a high gate-leakage current (bad devices). The apparent zero-bias Schottky barrier height of bad devices (0.4 < ϕB0 < 0.62 eV) was found to be lower than that of the good devices (ϕB0 = 0.79 eV). From transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analysis, we found that this difference is due to the presence of carbon impurities in the nickel layer in the gate region.

Temperature dependent transport characterization of iron on n-type (111) Si0.65Ge0.35 Schottky diodes

Temperature dependent transport characterization of Fe/n-Si0.65Ge0.35 Schottky diodes are investigated using current-voltage measurements between 60 K and 280 K. As temperature is decreased, ideality factor increased and apparent barrier height decreased. Following this trend, charge carriers transport through Metal-Semiconductor (MS) interface changes from Thermionic Emission (TE) to Thermionic Field emission (TFE) over barrier height. In addition, evidence is presented about lateral barrier height inhomogeneity at MS contact. Barrier height ϕB0 vs (1/2 kT) plots show double Gaussian distributions (GD) each with its mean barrier height and standard deviation. Modified Richardson plots obtained using this double GD lead to Richardson constant (A*) and ϕB0 in good agreement with theoretical values for Si1-xGex. Interface states density is also strongly temperature dependent indicating some temperature driven restructuring and reordering taking place at MS interface.

CURRENT–VOLTAGE CHARACTERISTICS OF THERMALLY ANNEALED Ni/n-GaAs SCHOTTKY CONTACTS

The Schottky barrier type Ni/[Formula: see text]-GaAs contacts fabricated by us were thermally annealed at 600[Formula: see text]C and 700[Formula: see text]C for 1[Formula: see text]min. The apparent barrier height [Formula: see text] and ideality factor of the diodes were calculated from the forward bias current–voltage characteristic in 60–320[Formula: see text]K range. The [Formula: see text] values for the nonannealed and 600[Formula: see text]C and 700[Formula: see text]C annealed diodes were obtained as 0.80, 0.81 and 0.67[Formula: see text]eV at 300[Formula: see text]K, respectively. Thus, it has been concluded that the reduced barrier due to the thermal annealing at 700[Formula: see text]C promises some device applications. The current preferentially flows through the lowest barrier height (BH) with the temperature due to the BH inhomogeneities. Therefore, it was seen that the [Formula: see text] versus [Formula: see text] plots for the nonannealed and annealed diodes showed the linear behavior according to Gaussian distributions.

A study of temperature dependent current–voltage (I–V–T) characteristics in Ni/sol–gel β-Ga2O3/n-GaN structure

β-Ga2O3 thin films were grown on n-type GaN substrates using the sol–gel method. The forward-biased temperature dependent current–voltage (I–V–T) characteristics of Ni/β-Ga2O3/GaN structure have been investigated in the temperature range of 298–473 K. The apparent barrier height ( $${\phi _{ap}}$$ ) increased while the ideality factor (n) decreased with the increase in temperature. Such a temperature dependent behavior of $${\phi _{ap}}$$ and n was explained by the inhomogeneity of $${\phi _{ap}}$$ , which obeyed Gaussian distribution with zero-bias mean barrier height ( $${\bar {\phi }_{B0}}$$ ) of 1.02 ± 0.02 eV and standard deviation ( $${\sigma _{s0}}$$ ) of 153 ± 0.04 mV. Subsequently, $${\bar {\phi }_{B0}}$$ and Richardson constant A* were obtained from the slope and intercept of the modified Richardson plot as 0.99 ± 0.01 e V and 67.2 A cm−2 K−2, respectively. The $${\bar {\phi }_{B0}}$$ obtained from the modified Richardson plot was in good agreement with the theoretical value calculated from the work function of Ni and electron affinity of β-Ga2O3. The I–V–T characteristics of Ni/β-Ga2O3/GaN MOS structures can be successfully explained by the thermionic emission theory with a single Gaussian distribution of the barrier height.

Self-assembled patches in PtSi/n-Si (111) diodes

Using the effect of the temperature on the capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics of PtSi/n-Si (111) Schottky diodes the profile of apparent doping concentration (NDapp), the potential difference between the Fermi energy level and the bottom of the conduction band (Vn), apparent barrier height (ΦBapp), series resistance (Rs) and the interface state density Nss have been investigated. From the temperature dependence of (C–V) it was found that these parameters are non-uniformly changed with increasing temperature in a wide temperature range of 79–360 K. The voltage and temperature dependences of apparent carrier distribution we attributed to the existence of self-assembled patches similar the quantum wells, which formed due to the process of PtSi formation on semiconductor and the presence of hexagonal voids of Si (111).

Gaussian Distribution of Inhomogeneous Barrier Height of Al/ZnS/ITO Schottky Barrier Diodes

The current–voltage (I–V) measurements on Al/ZnS Schottky barrier diodes in the temperature range 303 –423 K by the step of 15 K were carried out. The forward I–V characteristics were analyzed on the basis of the thermionic emission theory. The temperature dependence I-V parameters such as ideality factor (n) and barrier height (ϕb0) have been explained on the basis of inhomogeneity. An abnormal increase of apparent barrier height and decrease of ideality factor with increasing temperature have been explained due to the barrier height inhomogeneities on the basis of the thermionic emission theory with Gaussian distribution. Experimental results reveal the existence of a single Gaussian distribution with apparent barrier height value (ϕb0‾) of 1.091 eV and standard deviations (σs) of 0.18 V. Richardson constant (A*) was obtained as 8.49 x 10-2 A.m-2K-2 from the ln(I0/T2) vs. q/kT plot, which is far from the calculated value of 5.6 x 105 A.m-2K-2. The modified Richardson plotof ln(I0/T2) – (q2σs2/2k2T2) gives ϕb0‾ and A* values as 1.093 eV and 6.07 x 105 A.m-2K-2, without using the temperature coefficient of the barrier height. This obtained value of A* is extremely close to the previously calculated value. So, the temperature dependence of the forward bias I-V characteristics of the Schottky device can be successfully explained on the basis of the thermionic emission mechanism with a single Gaussian distribution of the barrier heights.

Effect of Insertion of Ultrathin Al2O3 Interlayer at Metal/GaN Interfaces

Fermi level depinning at a metal/semiconductor interface by using an ultrathin insulating interlayer to block the penetration of the metal wave function into the semiconductor is examined on GaN. A Si‐doped n‐type GaN epitaxial layer on a freestanding GaN substrate is used as the host material. For the samples with an interlayer, an ultrathin Al2O3 layer of 1 nm thickness is deposited by atomic layer deposition. As the metal layers, Ag, Cu, Au, Ni, and Pt are deposited by electron beam evaporation. Samples without an interlayer are also fabricated for comparison. The apparent change in current–voltage characteristics of the Schottky barrier diodes with the insertion of the interlayer is dependent on the electrode metal. However, the apparent Schottky barrier height (SBH) for the samples with the interlayer is almost constant and independent of the metal electrode, although the samples without an interlayer exhibit a moderate dependence of the SBH on the metal work function. Thus, the pinning becomes stronger upon the insertion of the interlayer, although blocking of the metal wave function by using an ultrathin insulator is expected to lead to depinning.

Numerical simulations of the electrical transport characteristics of a Pt/n-GaN Schottky diode

In this paper, using a numerical simulator, we investigated the current–voltage characteristics of a Pt/n-GaN thin Schottky diode on the basis of the thermionic emission (TE) theory in the 300 to 500 K temperature range. During the simulations, the effect of different defect states within the n-GaN bulk with different densities and spatial locations is considered. The results show that the diode ideality factor and the threshold voltage decrease with increasing temperature, while at the same time, the zero-bias Schottky barrier height (Φb0) extracted from the forward current density–voltage (J–V) characteristics increases. The observed behaviors of the ideality factor and zero-bias barrier height are analyzed on the basis of spatial barrier height inhomogeneities at the Pt/GaN interface by assuming a Gaussian distribution (GD). The plot of apparent barrier height (Φb,App) as a function of q/2kT gives a straight line, where the mean zero-bias barrier height () and the standard deviation (σ0) are 1.48 eV and 0.047 V, respectively. The plot of the modified activation energy against q/kT gives an almost the same value of and an effective Richardson constant A* of 28.22 A cm−2 K−2, which is very close to the theoretical value for n-type GaN/Pt contacts. As expected, the presence of defect states with different trap energy levels has a noticeable impact on the device electrical characteristics.

A DFT Exploration of Efficient Catalysts Based on Metal‐Salen Monomers for the Cycloaddition Reaction of CO2 to Propylene Oxide

AbstractUsing quantum chemistry calculations, we explored the interactions between non‐rare earth metal‐salophen complex (M‐Salen) monomers and propylene oxide (PO) and explored the catalytic mechanisms for the cycloaddition reaction of CO2 to epoxides by the M‐Salen monomers. Our theoretical results demonstrated that a larger binding energy for M‐Salen interacting with PO results in a lower apparent barrier height for the cycloaddition reaction. The CrIII‐Salen monomer possessed the lowest apparent barrier, while the AlIII‐Salen monomer showed the highest barrier along the reaction pathway, which are consistent with experimental results. In addition, a low apparent barrier was found for the ScIII‐Salen monomer, suggesting another potential conjugated microporous polymer catalyst for CO2 conversion.

Comparative Investigation on the Effects of Organic and Inorganic Interlayers in Au/n-GaAs Schottky Diodes

There have been many attempts on Schottky barrier diodes (SBDs) to improve the quality of metal–semiconductor (MS) structure and control permanently the barrier height (BH) by utilizing an organic or inorganic interfacial layer instead of widely used SiO2. The organic polymer or inorganic insulator interfacial layer insertion to the M/S interface converts the structure into the metal–polymer–semiconductor (MPS) or metal–insulator–semiconductor (MIS). The reliability and performance of the MIS/MPS-type SBDs is drastically affected by the interface layer quality and also its thickness. In this paper, MS structure forms as Au/n-GaAs are consecutively used with an organic interfacial layer polyvinyl alcohol and inorganic interfacial layer zinc oxide to compare their effects on the electrical characteristics of SBDs under distinct illumination levels. Consequently, the current-voltage characteristics provide us to reveal and compare the improvement and reliability of the MPS and MIS-type SBDs by also considering significant electrical parameters, such as ideality factor, apparent BH, and series resistance.

Study of radiative heat transfer in \xc5ngstr\xf6m- and nanometre-sized gaps

Radiative heat transfer in Ångström- and nanometre-sized gaps is of great interest because of both its technological importance and open questions regarding the physics of energy transfer in this regime. Here we report studies of radiative heat transfer in few Å to 5 nm gap sizes, performed under ultrahigh vacuum conditions between a Au-coated probe featuring embedded nanoscale thermocouples and a heated planar Au substrate that were both subjected to various surface-cleaning procedures. By drawing on the apparent tunnelling barrier height as a signature of cleanliness, we found that upon systematically cleaning via a plasma or locally pushing the tip into the substrate by a few nanometres, the observed radiative conductances decreased from unexpectedly large values to extremely small ones—below the detection limit of our probe—as expected from our computational results. Our results show that it is possible to avoid the confounding effects of surface contamination and systematically study thermal radiation in Ångström- and nanometre-sized gaps.

Schottky barrier height of Ni to β-(AlxGa1−x)2O3 with different compositions grown by plasma-assisted molecular beam epitaxy

Coherent β-(AlxGa1−x)2O3 films (x = 0, 0.038, 0.084, 0.164) were grown successfully on a Sn-doped β-Ga2O3 (010) substrate using plasma-assisted molecular beam epitaxy. Atom probe tomography, transmission electron microscopy, and high resolution x-ray diffraction were used to verify the alloy composition and high quality of the films. Schottky diodes were then fabricated using Ni as the Schottky metal. Capacitance–voltage measurements revealed a very low (<7 × 1015 cm−3) free charge density in the nominally undoped films. The barrier height and ideality factor were estimated by current–voltage (I–V) measurements performed at temperatures varying from 300 K to 500 K on the Schottky diodes. These measurements revealed that the apparent Schottky barrier height could have similar values for different compositions of β-(AlxGa1−x)2O3. We believe this is attributed to the lateral fluctuation in the alloy’s composition. This results in a lateral variation in the barrier height. Therefore, the average Schottky barrier height extracted from I–V measurements could be similar for β-(AlxGa1−x)2O3 films with different compositions.

Metal-insulator-SiC Schottky structures using HfO2 and TiO2 dielectrics

Metal-Insulator-Semiconductor Schottky diodes were fabricated on SiC, as a potential use for particle detectors. Nickel was used as Schottky and back ohmic contacts. The dielectrics HfO2 and TiO2 were investigated as insulating layers and deposited by Atomic Layer Deposition, with thicknesses of 1, 2 and 4nm. Current-Voltage curves were extracted from the diodes, varying the measurement temperature (297K–373K). Apparent and real Schottky Barrier Heights (SBHapparent and SBHreal), ideality factor η and insulating layer thicknesses were extracted from the I-V curves. Thicker insulating layers produce higher η and reduce the SBHreal value, for both dielectrics. An interfacial layer of silicon oxycarbide with thickness of 0.2nm was estimated for all diodes. The SBHreal goes from 1.22V to 0.66V and from 1.26 V to 0.59 V, for thicknesses of 1nm and 4nm of HfO2 and TiO2, respectively. The reverse currents for all structures at 40V of bias are of order of tens of pA at room temperature.

Investigation of barrier inhomogeneities and interface state density in Au/MgZnO: Ga Schottky contact

The electrical characteristics of Au/MgZnO:Ga (GMZO) Schottky contact, which is fabricated using a dual ion beam sputtering system, have been investigated using current–voltage (I–V) and capacitance–voltage (C–V) measurements over a wide temperature range of 80 to 300 K. The apparent Schottky barrier height (SBH) and ideality factor obtained from the I–V measurements are observed to increase and reduce, respectively, with increasing measurement temperature. This anomalous observation in the behaviour of the SBH is in good agreement with the predictions of a double Gaussian distribution (DGD) of the inhomogeneous SBH at a metal–semiconductor (MS) interface. The values of the SBH as determined from C–V measurements are expectedly higher than those extracted from I–V measurements. The DGD model is observed to fit with experimentally obtained data for the temperature-dependent SBH with mean values of the SBH of 0.95 and 0.54 eV and standard deviations of 0.131 and 0.072 eV in the temperature range of 160–300 K and 80–160 K, respectively. The larger value of the SBH standard deviation confirms more SBH inhomogeneity at the MS interface, and these inhomogeneities are attributed to the presence of deep level or surface level interface states. The calculated interface states density is seen to vary from 6.46 × 1014 eV−1 cm−2 at EC-0.27 eV to 1.58 × 1014 eV−1 cm−2 at EC-0.74 eV, where EC is the bottom of a conduction band at 300 K.

COMPARISON OF THE Ti/n-GaAs SCHOTTKY CONTACTS’ PARAMETERS FABRICATED USING DC MAGNETRON SPUTTERING AND THERMAL EVAPORATION

We have fabricated the Ti/[Formula: see text]-type GaAs Schottky diodes (SDs) by the DC magnetron deposition and thermal evaporation, cut from the same GaAs substrates, and we have made a comparative study of the current–voltage ([Formula: see text]–[Formula: see text]) measurements of both SDs in the measurement temperature range of 160–300[Formula: see text]K with steps of 10[Formula: see text]K. The barrier height (BH) values of about 0.82 and 0.76[Formula: see text]eV at 300[Formula: see text]K have been obtained for the sputtered and evaporated SDs, respectively. It has been seen that the apparent BH value for the diodes has decreased with decreasing temperature obeying the single-Gaussian distribution (GD) for the evaporated diode and the double-GD for the sputtered diode over the whole measurement temperature range. The increment in BH and observed discrepancies in the sputtered diode have been attributed to the reduction in the native oxide layer present on the substrate surface by the high energy of the sputtered atoms and to sputtering-induced defects present in the near-surface region. We conclude that the thermal evaporation technique yields better quality Schottky contacts for use in electronic devices compared to the DC magnetron deposition technique.

In silico simulations of tunneling barrier measurements for molecular orbital-mediated junctions: A molecular orbital theory approach to scanning tunneling microscopy

A new simulator for scanning tunneling microscopy (STM) is presented based on the linear combination of atomic orbitals molecular orbital (LCAO-MO) approximation for the effective tunneling Hamiltonian, which leads to the convolution integral when applied to the tip interaction with the sample. This approach intrinsically includes the structure of the STM tip. Through this mechanical emulation and the tip-inclusive convolution model, dI/dz images for molecular orbitals (which are closely associated with apparent barrier height, ϕap) are reported for the first time. For molecular adsorbates whose experimental topographic images correspond well to isolated-molecule quantum chemistry calculations, the simulator makes accurate predictions, as illustrated by various cases. Distortions in these images due to the tip are shown to be in accord with those observed experimentally and predicted by other ab initio considerations of tip structure. Simulations of the tunneling current dI/dz images are in strong agreement with experiment. The theoretical framework provides a solid foundation which may be applied to LCAO cluster models of adsorbate–substrate systems, and is extendable to emulate several aspects of functional STM operation.